1. Field of the Invention
This invention relates to transient voltage surge suppression for low-voltage signal circuits. More particularly, this invention relates to protecting low-voltage signal circuits which receive data or signals from an unbalanced coaxial cable from transient voltage surges by means of a circuit comprising a gas tube connected across the shield and signal conductors of the cable in combination with a metal oxide varistor connecting the shield conductor to ground. In another embodiment wherein the data signal is received from a twin lead balanced line, a first gas tube will be connected across both leads, a second gas tube will connect one of the signal leads to ground and a third gas tube will connect the other signal lead to ground.
2. Background of the Disclosure
It has been determined that transient voltage surge suppressors are needed to protect consumer electronic equipment now commonly found in many households. It is also necessary to provide protection from transient voltage surges for low-voltage data circuits, such as those found on line-powered products that use an antenna or other line connection. Such antenna line connections include an unbalanced line such as a coaxial cable which provides low-voltage data signals to VCR's, television sets, home computers, telephonic electronic equipment, etc., as well as with balanced lines such as the common 300 ohm flat twin lead antenna wire employed with television sets. Low-voltage data receiving and processing circuits and associated equipment are extremely susceptible to damage and destruction by very short duration transient voltage spikes. A characterization of these spikes is provided by the Institute of Electrical and Electronic Engineers, IEEE Standard 587-1980. This document is publicly available and describes voltage surges with durations of typically about 28 microseconds, peak voltages of about 6000 volts and peak follow currents of about 3000 amperes. Even though such surges are generally of short duration, a cumulative effect will cause failure of the electronic components in the data receiving circuit.
Present ways of accomplishing transient voltage surge suppression for low voltage data circuits include a gas tube connected across the shield and signal conductors of a signal containing coaxial cable as is shown schematically in FIG. 1(a). Thus, gas tube 10 is connected across shield conductor 12 and data signal conductor 14 of coaxial cable 16. Moreover, the gas tube 10 connected across the data signal line is used in conjunction with a power surge suppression circuit such as that schematically shown in FIG. 1(b). The transient power voltage surge suppression circuit shown in FIG. 1(b) is a conventional type employed with a common household outlet having 3 conductors: line 18, neutral 20 and ground 22. The power voltage surge suppression circuit comprises a first metal oxide varistor (MOV) 24 connected across line 18 and neutral 20 and a second MOV 26 connected across neutral 20 and ground 22. Power surges may be generated from line to neutral, line to ground or neutral to ground.
Another method of transient voltage surge suppression is the type generally employed with telephone line data connections schematically shown in FIG. 1(c). In this circuit, separate gas tubes 28 and 30 independently connect conductors 13 and 15, respectively, to the ground line 22 of a power surge suppressor such as the type discussed above which comprises two MOV's 24 and 26 which separately connect the line 18 and ground 22 conductors to neutral conductor 20.